Apache NetBeans (incubating) 9.0 Beta is the Apache NetBeans incubator release of the NetBeans Java SE IDE, i.e., the NetBeans Platform that underpins NetBeans IDE, released as Apache NetBeans (incubating) 9.0 Alpha, together with all the features and tools applicable to general Java development – including Java Swing, JavaFX, and the support created specifically for Java 9, i.e., JShell, JLink, and Jigsaw.

Tasks to be completed for Apache NetBeans (incubating) 9.0 Beta to be released:

define a mechanism by means of which nb-javac is installed by the user, on demand, since it cannot be installed by default because it is GPL licensed: "Any NetBeans user who wants to use the optional Java module will have to separately install the nb-javac module. The Apache NetBeans project will provide instructions and/or helpful scripts to assist in the installation. The user will be required to accept the terms of the license."
LEGAL-279
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Getting issue details...STATUS

Solution: at startup, this dialog is shown, when running on JDK 8:

When running on JDK 9, a notification appears, in Window | IDE Tools | Notifications, via which nb-javac can be installed, as an alternative to using javac directly from JDK 9.

...when "Install nb-javac" is clicked, the user needs to explicitly accept the terms of the license: